The present invention relates to improvement in fractionation of close boiling volatile components, especially cryogenic separation of light gases. In particular it relates to a new method for recovering ethene (ethylene) or propene (propylene) from cracking gas or the like in mixture with other components requiring low temperature refrigeration.
Cryogenic technology has been employed on a large scale for recovering gaseous hydrocarbon components, such as C.sub.1 -C.sub.2 alkanes and alkenes from diverse sources, including natural gas, petroleum refining, coal and other fossil fuels. Separation of high purity ethene from other gaseous components of cracked hydrocarbon effluent streams has become a major source of chemical feedstocks for the plastics industry. Polymer grade ethene, usually containing less than 1% of other materials, can be obtained from numerous industrial process streams. Thermal cracking and hydrocracking of hydrocarbons are employed widely in the refining of petroleum and utilization of C.sub.2.sup.+ condensible wet gas from natural gas or the like. Low cost hydrocarbons are typically cracked at high temperature to yield a slate of valuable products, such as pyrolysis gasoline, lower olefins and LPG, along with byproduct methane and hydrogen. Conventional separation techniques near ambient temperature and pressure can recover many cracking effluent components by sequential liquefaction, distillation, sorption, etc. However, separating methane and hydrogen from the more valuable C.sub.2.sup.+ aliphatics, especially ethene, ethane, propene, and/or propane requires relatively expensive equipment and processing energy. Primary emphasis herein is placed on a typical large scale cryogenic plant for recovering ethene from cracking gas.
Typical cryogenic systems are described in U.S. Pat. Nos. 3,126,267 (VanderArend); 3,702,541 (Randall et al); 4,270,940 (Rowles et al); 4,460,396 (Kaiser et al); 4,496,380 (Harryman); 4,368,061 (Mostrail et al); and 4,900,347 (McCue et al).
It is an object of the present invention to provide an improved cold fractionation system for separating light gases at low temperature which is energy efficient and saves capital investment in cryogenic equipment.